1. Technical Field
The present invention relates to a quantum interference device, an atomic oscillator, and a magnetic sensor, and, more particularly to a technique for efficiently causing an EIT phenomenon.
2. Related Art
An atomic oscillator employing an electromagnetically induced transparency system (EIT system, which may be called CPT system) is an oscillator that makes use of a phenomenon in which, when two resonant lights having different wavelengths are simultaneously irradiated on alkali metal atoms, absorption of the two resonant lights stops (an EIT phenomenon). FIG. 24A is a diagram of an energy state of one alkali metal atom. When first resonant light having wavelength equivalent to an energy difference between a first ground level 23 and an excitation level 21 or second resonant light having wavelength equivalent to an energy difference between a second ground level 24 and an excitation level 21 is independently irradiated on alkali metal atoms, as it is well known, light absorption occurs. However, when the first resonant light and the second resonant light are simultaneously irradiated on the alkali metal atoms and a frequency difference between the simultaneously-irradiated first and second resonant lights precisely coincides with an energy difference (ΔE12) between the first ground level 23 and the second ground level 24, a system shown in FIG. 24A changes to a superimposed state of the two ground levels, i.e., a quantum interference state. Therefore, excitation to the excitation level 21 stops and a transparency (EIT) phenomenon occurs. It is possible to manufacture a highly-accurate oscillator by detecting, as a signal, a steep change in light absorption behavior at the time when a wavelength difference between the first resonant light and the second resonant light deviates from ΔE12 and controlling the signal making use of this phenomenon. Since ΔE12 sensitively changes because of the intensity or fluctuation of external magnetism, it is also possible to manufacture a highly-sensitive magnetic sensor making use of the EIT phenomenon.
To improve a signal-to-noise ratio (S/N) of an optical output signal due to the EIT phenomenon, the number of atoms of alkali metal, which interacts with resonant light, only has to be increased. For example, JP-A-2004-96410 (Patent Document 1) discloses, for the purpose of improving an S/N of an output signal of an atomic oscillator, a method of increasing the thickness of a cell in which gaseous alkali metal atoms are confined and a method of increasing a beam diameter of a laser beam made incident on the cell. In both the methods, to increase an area where the alkali metal atoms come into contact with resonant light, the thickness or the height of the cell is increased as shown in FIG. 24B or FIG. 24C. As the laser beam, only a pair of laser beams having two kinds of wavelength that satisfy a development condition for the EIT phenomenon are used.
U.S. Pat. No. 6,359,916 (Patent Document 2) discloses (1) a technique concerning improvement of the sensitivity of an EIT (CPT) system atomic oscillator. The technique has a characteristic that a D1 line is used as a light source. Theoretically, EIT (CPT) signal intensity can be improved compared with that in the case of a D2 line in the past. Consequently, sensitivity and frequency stability is improved. (2) The signal intensity is further improved by using a four-wave light source and causing P1/2 excitation levels (hyperfine structure), which are energy-split into two, to simultaneously interact in a double Λ type transition. The technique disclosed in the patent document relates to four wave mixing, which is on the outside of the range of the technical field related to the invention.
When attention is paid to respective atoms forming a group of gaseous alkali metal atoms in a cell, the atoms have fixed velocity distribution corresponding to motion states thereof. When laser beams having only two kinds (a pair of) wavelengths are made incident on the atom group, because of the influence of the Doppler effect (Doppler shift) due to the motion of the atoms, atoms that can actually interact one another are limited to only a part of atoms having values of specific velocity components with respect to a laser incident direction among a large number of atoms in the cell. Therefore, a ratio of atoms contributing to the EIT development is extremely low. The related art disclosed in Patent Document 1 is the atomic oscillator configured in such a state of low EIT development efficiency. Therefore, to obtain a desired absorption spectrum with a large signal-to-noise ratio (S/N), the thickness or the height of the cell has to be increased. It is difficult to reduce the size of the cell while maintaining the signal-to-noise ratio. In other words, the number of atoms contributing to the EIT phenomenon per a unit volume in the cell remains the same. The technique disclosed in Patent Document 2-(1) has the same problem.
Specifically, in both Patent Document 1 and Patent Document 2-(1), only two light waves are used. Since the alkali metal atoms in the cell have a velocity distribution, Doppler broadening of energy involved in the velocity distribution is present. Therefore, since only a part of atoms interact with one another in Λ type transition of only the two light waves, an EIT development yield per a unit volume is extremely poor. Therefore, EIT signal intensity is low.
An excitation level of existing alkali metal atoms has a hyperfine structure and is split into levels having different energies as shown in FIGS. 20A to 20C. Therefore, since the EIT phenomenon targeting the alkali metal atoms cannot be explained in a simple Λ type three-level system shown in FIG. 24A, to actually cause EIT efficiently, it is necessary to take into account such multiple levels. However, so far, sufficient examination has not been made by taking into account a relation between the presence of the multiple level and the Doppler broadening of energy involved in the atomic velocity distribution.
In particular, as in the invention, when plural resonant light pairs are used, it is important in terms of optimization of a driving condition for a quantum interference device employing the EIT phenomenon to determine a center frequency of a light source (a laser) taking into account an energy state of an excitation level and determine a modulation condition for the laser.